The present invention relates to a pressure control system for stern tube seals, and more particularly to a system for controlling the pressure within annular chambers defined by adjacent seal rings in stern tube seals using jointly compressed air and pressurized oil.
Since stern tube seals of a radial lip type formed of resilient material have come to be used in general ships, the pressure applied to the seal rings have increased due to the tendency toward the use of large ships with resultant increase in the draft pressure. Consequently, the number of troubles in which the seal rings become damaged have increased, and the resultant leakage of oil to the outboard of ships presents the problem of environmental pollution. Hence, various proposals have hitherto been made to cope with this problem. In such proposals, the basic technological concept has been that the oil pressure inboard the stern tube should be set at a level higher (approx. 0.3 kg/cm.sup.2 f) than the seawater pressure, thereby ultimately preventing the ingress of seawater into the stern tube. This concept has been adopted as a basic rule since the invention of the stern tube seals. Accordingly, in the conventional stern tube seals, a system for automatically adjusting the lubricating oil pressure, which is adapted to vary the lubricating oil pressure within the stern tube in response to the fluctuations of the draft, is arranged as follows: an air purge chamber is provided at the vicinity of centerline of the tail shaft to detect a pressure corresponding to the draft with a sensor, the detected signal is then converted into a pneumatic pressure with a differencial pressure transmitter to actuate a pressure regulator, a regulating valve, etc., thereby adjusting the pressure within the stern tube.
In this system, however, the various devices themselves constituting the system are very complicated and expensive. In addition, a time lag is caused in operation, which gives rise to a phase differential in pressure fluctuations between the respective adjacent annular chambers, resulting in leakage of oil into seawater, which has been a major problem in environmental pollution. In addition, although small stern tube seals have also been used which make use of compressed air as a pressure medium for the annular chamber between the adjacent seal rings, the flow rate of compressed air leaking to the outboard tends to be large, which results in an increased amount of compressed air consumed. Hence, there have been such problems as excessive power loss and maintenance expense being required.